Switchover is an important feature of media gateways to support scheduled in-service upgrade and to handle unexpected card failures. When a working service module or card of a media gateway fails, the media gateway must include provisions to move or “switchover” active channels onto a backup module or card. Moreover, it is desirable that the switchover be transparent to the remote end of any active calls, but this can be hard to achieve in some cases, such as in IuUP-based calls used in UMTS networks.
That is, at least according to the standard IuUP protocol state machine, a UMTS IuUP call instance cannot be transparently switched from one service card over to another service card because, to transit from the Null state to the Transfer Data Ready state, an IuUP call instance must go through the Initialization state, and must send out a new IuUP INIT message to the remote peer and receive a positive acknowledgement.
There is also a need in this area of technology for significantly reducing call drop rates and increasing service provider revenue. For example, when a packet (IP or ATM) network has high packet loss, the IuUP/NbUP INIT messages to initiate a UMTS call are often lost, and cannot reach the far-end peer. One or more retries may be attempted but, eventually, after a configured timer expires without receiving any positive acknowledgement message, the local end stops sending the INIT messages, and the call is dropped. In another scenario, when the local end starts to send INIT messages but the far-end has not yet been set up, the INIT messages sent by the local end will be dropped by the far end. Again, one or more retries may be attempted but, eventually, after a configured timer expires without receiving any positive acknowledgement message, the local end stops sending the INIT messages, and the call is dropped.
The standard UMTS IuUP protocol fails to handle these scenarios properly because it does not differentiate between the “NEGATIVE Acknowledgement” case, which means a definitive negative response, and the “No Acknowledgement” case, which means lack of any positive or negative response and may need additional time for retries. A single timer is used for both cases, which is usually small in order to speed up the decision at the “NEGATIVE Acknowledgement” case, but not sufficiently long to accommodate the “No Acknowledge” case.